Yall every time I think about (think about, not even watch, just think about!) the final 15, I find new things about Aziraphale's point of view that I think many people are missing
This is the face of an angel who just realised that all this time heaven had been fully capable of "looking through his previous exploits" and seeing clear as day that he's been working with a demon this whole time.
If this doesn't make him feel threatened (on his own and Crowley's behalf), then I don't know what else it does. After all, this is what they had been trying to avoid for 6000 years. The Arrangement worked, because neither of their Head Offices cared enough to check how things got done.
And now it turns out someone did...
Now they could get punished for their centuries of their illegal cooperation...
Now tell me, do you still argue with the man who has such an argument on his side, or do you try and mitigate the conflict and possible bloodshed (after all, isn't he also like the second most powerful being in the entire universe? And when he smites you, you've been... you know?)?
Because if I know one thing about Aziraphale, it's that he will try to solve problems by talking first thing. He won't back down an inch when a fight starts, but first he'll walk a mile to avoid it at any cost.
And this might be the biggest fight of his existence.
There didn't have to be a hidden part of the conversation with Metatron; a scene we didn't see. There didn't necessarily have to be any "come to heaven or else". This was enough of a threat. And Aziraphale is intelligent. He knows it.
He knows he is basically at gunpoint...